Serna Horacio(first author), Barriuso G. C. Miguel, Pagonabarraga Ignacio, Polin Marco & Valeriani Chantal. Sorting of binary active-passive mixtures in designed microchannels. Soft Matter 20th Anniversary Collection. 2025

In this article, we study mixtures of active and passive particles confined in designed microchannels that feature funnel-like obstacles in their central part. The obstacles comprise gaps that only allow the passage of the passive particles, thus behaving like a sempermeable membrane. We show that by tunning the tumbling rate of the active particles and changing the tilt angle of the obstacles, the system reaches a maximum in performance in terms of the maximum separation efficiency and the shortest time response.  We demmonstrate that the reason behind such top performance is  an activity-induced advective drift on the passive particles.  This mechanism allows what we call active-pumping, a very intresting desing principle that might be exploited in the development of separation and cargo delivery systems at the microscale, harenssing microorganisms motility.

arXiv

Soft Matter

Martin-Roca Jose, Escobar Ortiz Daniel, Valeriani Chantal & Serna Horacio*(leading and corresponding author). Confinement-induced collective motion in suspensions of run-and-tumble particles. Submitted to Journal of Chemical Physics

In this article,  we study the behaviour of an active suspension confined in michorcahnnels with assymetric boundaries: the top boundary is a flat wall and the bottom boundary is an array of funnel-like obstacles featruing gaps that ovoid the passage of the active particles. To the best of our knowledge, we report for the first time the emergence of a dense traveling band whose formation and motion are completely induced and sustained by confinement, with neither explicit alignment between agents, nor effective alignment induced by particle shape or collisions, nor by the presence of passive particles in the suspension. We discovered such a traveling structure serendipitously, while performing the analysis of the results published in Serna et. al. , Soft Matter, 2025,  by some of the present authors. The mechanism of the band's motion resembles the tracked locomotion of some heavy vehicles such as tractors, and thus we have named it confinement-induced tracked locomotion. This discovery might be realized experementally by confining suspensions of Chlamydomonas reinhardtii into ring-shaped microfluidic chambers and might be exploited in the one-dimensional transport of passive micro objects.

arXiv